Polyolefins are a class of polymers derived from simple olefins. Known methods of making polyolefins involve the use of Ziegler-Natta polymerization catalysts. These catalysts polymerize vinyl monomers using a transition metal halide to provide a polymer with an isotactic stereochemical configuration.
Basically two types of Ziegler-Natta catalyst systems are used in the normal processes for the polymerization or copolymerization of olefins. The first one, in its broadest definition, comprises TiCl3 based catalysts components, obtained for example by reduction of TiCl4 with Al-alkyls, used in combination with Al-compounds such as diethylaluminum chloride (DEAC). Despite the modest properties of the polymers in terms of isotacticity the catalysts are characterized by a very low activity which causes the presence of large amounts of catalytic residues in the polymers.
The second type of catalyst system comprises a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound. In order to maintain the high selectivity for an isotactic polymer product, a variety of internal electron donor compounds must be added during the catalyst synthesis. Conventionally, when a higher crystallinity of the polymer is required, also an external donor compound is added during the polymerization reaction. Both the internal and external electron donor compounds become indispensable compositions of catalyst components.
During the past 30 years, numerous supported Ziegler-Natta catalysts have been developed which afford a much higher activity in olefin polymerization reactions and much higher contents of crystalline isotactic fractions in the polymers they produce. With the development of internal and external electron donor compounds, polyolefin catalyst system is continuously renovated.